Pump/Motor seals contain the water in the pump and prevent intrusion into the motor. Please refer to your pump manual or pump breakdown to select the proper seal. Viton seals are much more durable in aggressive water (highly recommended in salt pools).
When installing a pump/motor seal make sure to put the rubber with porcelain half into the motor seal plate and the plastic end of the spring seal towards the porcelain. The motor seal MUST be replaced along with the motor, because installing a replacement motor with an old pump can cause the pump to leak which can lead to a variety of problems.